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Help Net Security: A Quick Way to Secure a Linux System

"The first step in securing your system is to determine what the
function of that system is going to be. For example, if the system
is going to be dedicated to being an ftp server,then there is
little reason to having samba or sendmail installed because the
more services that are running, the more vulnerable the system is
going to be. This cannot be stressed enough. You do not want
services running that you won't be using.

"So we begin with a fresh install of the system with the Linux
distribution of your choice, and in that installation process we'll
choose the security settings for 'High' or whatever the equivalent
is if the option is available. This should enable package
filtering, regulating what is and isn't allowed to connect to your
system..."